Silica-based sols

ABSTRACT

A process for the production of an aqueous sol containing silica-based particles which comprises (a) acidifying an aqueous silicate solution to a pH of from 1 to 4 to form an acid sol; (b) alkalizing the acid sol at an SiO 2  content within the range of from 4.5 to 8% by weight; (c) allowing particle growth of the alkalized sol for at least 10 minutes; or heat-treating the alkalized sol at a temperature of a least 30° C.; (d) alkalizing the obtained sol to a pH of at least 10.0; and (e) optionally concentrating the sol obtained according to (b), (c) or (d) to provide an aqueous sol containing silica-based particles and having a specific surface area of at least 90 m 2 /g aqueous sol; as well as an aqueous sol containing silica-based particles obtainable by the process. The invention also relates to an aqueous sol containing silica-based particles which sol has a specific surface area of at least 115 m 2 /g aqueous sol and an S-value within the range of from 10 to 45% or contains silica-based particles having a specific surface area of at least 550 and less than 1000 m 2 /g SiO 2 . The invention further relates to the use of the aqueous sol containing silica-based particles as a drainage and retention aid in the production of paper as well as a process for the production of paper from an aqueous suspension containing cellulosic fibers, and optional filler, in which silica-based particles and at least one charged organic polymer are added to the cellulosic suspension.

[0001] The present invention generally relates to silica-based solssuitable for use in papermaking. More particularly, the inventionrelates to silica-based sots and silica-based particles, theirproduction and their use in the production of paper. The process of thisinvention provides silica-based particles and sols containingsilica-based particles with high drainage and retention performance,high stability and high solids contents.

BACKGROUND

[0002] In the papermaking art, an aqueous suspension containingcellulosic fibres, and optional fillers and additives, referred to asstock, is fed into a headbox which ejects the stock onto a forming wire.Water is drained from the stock through the forming wire so that a wetweb of paper is formed on the wire, and the paper web is furtherdewatered and dried in the drying section of the paper machine. Drainageand retention aids are conventionally introduced into the stock in orderto facilitate drainage and to increase adsorption of fine particles ontothe cellulosic fibres so that they are retained with the fibres on thewire.

[0003] Silica-based particles are widely used as drainage and retentionaids in combination with charged organic polymers like anionic andcationic acrylamide-based polymers and cationic and amphoteric starches.Such additive systems are disclosed in U.S. Pat. Nos. 4,388,150;4,961,825; 4,980,025; 5,368,833; 5,603,805; 5,607,552; and 5,858,174;and International Patent Application WO 97/18351. These systems areamong the most efficient drainage and retention aids now in use.

[0004] Silica-based particles suitable for use as drainage and retentionaids are normally supplied in the form of aqueous colloidal dispersions,so-called sols. Commercially used silica-based sols usually have asilica content of about 7 to 15% by weight and contain particles with aspecific surface area of at least 300 m²/g. Sots of silica-basedparticles with higher specific surface areas are usually more dilute toimprove storage stability and avoid gel formation.

[0005] It would be advantageous to be able to provide silica-based solsand particles with further improved drainage and retention performanceand even better stability. It would also be advantageous to be able toprovide a process for preparing silica-based sols and particles withimproved drainage, retention and stability properties. It would also beadvantageous to be able to provide a papermaking process with improveddrainage and/or retention.

THE INVENTION

[0006] In accordance with the present invention there are providedsilica-based sols and particles which are suitable for use asflocculating agents in water purification and as drainage and retentionaids in papermaking. The silica-based sols and particles according tothe invention exhibit good stability over extended periods of time,notably high surface area stability and high stability towards gelation,and hence they can be prepared and shipped at high specific surfaceareas and high silica concentrations. The silica-based sols andparticles have improved capability to maintain the high specific surfacearea on storage at high silica concentrations. The silica-based sols andparticles according to the invention further result in very good orimproved drainage and retention when used in conjunction with anionic,cationic and/or amphoteric organic polymers. Hereby the silica-basedsols and particles according to the invention makes it possible toincrease the speed of the paper machine and to use a lower dosage ofadditives to give a corresponding drainage and/or retention effect,thereby leading to an improved papermaking process and economicbenefits. The invention thus relates to silica-based particles and anaqueous sol containing silica-based particles, herein also referred toas silica-based sol, and their production, as further defined in theappended claims.

[0007] The present invention also relates to the use of the silica-basedsols and particles as drainage and retention aids in papermaking,preferably in combination with organic polymers as described herein, asfurther defined in the appended claims. The term “drainage and retentionaid”, as used herein, refers to one or more components (aids, agents oradditives) which, when being added to a papermaking stock, give betterdrainage and/or retention than is obtained when not adding thecomponents. The present invention further relates to a process for theproduction of paper from an aqueous suspension containing cellulosicfibres, and optional fillers, which comprises adding to the suspensionsilica-based particles and at least one charged organic polymer, formingand draining the suspension on a wire. The invention thus relates to aprocess as further defined in the appended claims.

[0008] The silica-based sols according to the present invention areaqueous sols that contain anionic silica-based particles, i.e. particlesbased on silica (SiO₂) or silicic acid. The particles are preferablycolloidal, i.e., in the colloidal range of particle size. Thesilica-based particles present in the sol suitably have an averageparticle size below about 10 nm and preferably in the range of fromabout 10 to about 2 nm. As conventional in the chemistry of colloidalparticles based on silica, particle size refers to the average size ofthe primary particles, which may be aggregated or non-aggregated.

[0009] The specific surface area of the silica-based sols is suitably atleast 80 m²/g aqueous sol, i.e., based on the weight of aqueous sol,preferably at least 85 m²/g aqueous sol, more preferably at least 90m²/g aqueous sol and most preferably at least 95 m²/g aqueous sol. In apreferred embodiment of this invention, the specific surface area of theaqueous silica-based sols is suitably at least 115 m²/g aqueous sol,preferably at least 120 m²/g aqueous sol. Generally, the specificsurface area of the aqueous sol can be up to about 200 m²/g aqueous sol,suitably up to 150 m²/g aqueous sol and preferably up to 130 m²/gaqueous sol.

[0010] The specific surface area of the silica-based particles issuitably at least 300 m²/g Si0 ₂, i.e. based on the weight of SiO₂,preferably at least 400 m²/g SiO₂ and most preferably at least 550 m²/gSiO₂. Generally, the specific surface area of the particles can be up toabout 1200 m²/g SiO₂, suitably less than 1000 m²/g SiO₂ and preferablyup to 950 m²/g SiO₂. In a preferred embodiment of this invention, thespecific surface area of the particles is within the range of from 550to 725 m²/g SiO₂, preferably from 575 to 700 m²/g SiO₂. In anotherpreferred embodiment of this invention, the specific surface area of theparticles is within the range of from 775 to 1200 m²/g SiO₂, preferablyfrom 800 to less than 1000 m²/g SiO₂.

[0011] The specific surface area can be measured by means of titrationwith NaOH in known manner, e.g. as described by Sears in AnalyticalChemistry 28(1956):12, 1981-1983 and in U.S. Pat. No. 5,176,891, afterappropriate removal of or adjustment for any compounds present in thesample that may disturb the titration like aluminium and boron species.When expressed in square metres per gram of aqueous sol, the specificsurface area represents the specific surface area that is available pergram of aqueous silica-based sol. When expressed in square metres pergram of silica, the specific surface area represents the averagespecific surface area of the silica-based particles present in the sol.

[0012] The silica-based sols usually have an S-value within the range offrom 10 to 45%, suitably from 20 to 40% and preferably from 25 to 35%.The S-value can be measured and calculated as described by ILer & Daltonin J. Phys. Chem. 60(1956), 955-957. The S-value indicates the degree ofaggregate or microgel formation and a lower S-value is indicative of ahigher degree of aggregation.

[0013] The silica-based sols usually have a molar ratio of SiO₂ to M₂O,where M is alkali metal ion (e.g. Li, Na, K) and/or ammonium, of atleast 10:1, suitably at least 12:1 and preferably at least 15:1. Themolar ratio of SiO₂ to M₂O generally can be up to 100:1, suitably up to40:1 and preferably up to 30:1. Preferred ranges are thus from 10:1 to100:1 and notably from 15:1 to 30:1. The silica-based sols usually havea pH of at least 8.0, suitably at least 10.0, preferably at least 10.5,and more preferably at least 10.6. The pH can be up to about 11.5,suitably up to 11.0.

[0014] The silica-based sols should suitably have a silica content of atleast 3% by weight but it is more suitable that the silica content iswithin the range of from 10 to 30% by weight and preferably from 12 to25% by weight. In order to simplify shipping and reduce transportationcosts, it is generally preferable to ship high concentrationsilica-based sols but it is of course possible and usually preferable todilute and mix the silica-based sols and particles to substantiallylower silica contents prior to use, for example to silica contentswithin the range of from 0.05 to 5% by weight, in order to improvemixing with the furnish components. The viscosity of the silica-basedsols can vary depending on, for example, the silica content of the sol.Usually, the viscosity is at least 5 cP, normally within the range offrom 5 to 40 cP, suitably from 6 to 30 cP and preferably from 7 to 25cP. The viscosity, which is suitably measured on sols having a silicacontent of at least 10% by weight, can be measured by means of knowntechnique, for example using a Brookfield LVDV II+ viscosimeter.

[0015] The silica-based sols of this invention are preferably stable.The term “stable silica-based sol”, as used herein refers tosilica-based sols which when subjected to storage or ageing for onemonth at 20° C. in dark and non-agitated conditions exhibit an increasein viscosity of less than 100 cP. Suitably the viscosity increase, ifany, is less than 50 cP and preferably less than 30 cP when the sols aresubjected to the above conditions.

[0016] In a preferred embodiment of this invention, the silica-based solis substantially free from aluminium, i.e. free from added modifierscontaining aluminium. In another preferred embodiment of this invention,the silica-based sol is substantially free from boron, i.e. free fromadded modifiers containing boron. Minor amounts of such elements canhowever be present in the starting materials used to prepare thesilica-based sols and particles. In yet another preferred embodiment ofthis invention, the silica-based sols are modified using variouselements, e.g. aluminium and/or boron, which can be present in theaqueous phase and/or in the silica-based particles. If aluminium isused, the sols can have a molar ratio of Al₂O₃ to SiO₂ within the rangeof from 1:4 to 1:1500, suitably from 1:8 to 1:1000 and preferably from1:15 to 1:500. If boron is used, the sols can have a molar ratio of B toSiO₂ within the range of from 1:4 to 1:1500, suitably from 1:8 to 1:1000and preferably from 1:15 to 1:500. If both aluminium and boron are used,the molar ratio of Al to B can be within the range of from 100:1 to1:100, suitably from 50:1 to 1:50.

[0017] Silica-based sols and particles according to the invention can beproduced starting from a conventional aqueous silicate solution likealkali water glass, e.g. potassium or sodium water glass, preferablysodium water glass. The molar ratio of SiO₂ to M₂O, where M is alkalimetal, e.g. sodium, potassium, ammonium, or a mixture thereof, in thesilicate solution or water glass is suitably within the range of from1.5:1 to 4.5:1, preferably from 2.5:1 to 3.9:1. Suitably a dilutesilicate solution or water glass is used which can have an SiO₂ contentof from about 3 to about 12% by weight, preferably from about 5 to about10% by weight. The silicate solution or water glass, which usually as apH around 13 or above 13, is acidified to a pH of from about 1 to about4. The acidification can be carried out in known manner by addition ofmineral acids, e.g. sulphuric acid, hydrochloric acid and phosphoricacid, or optionally with other chemicals known as suitable foracidification of water glass, e.g. ammonium sulphate and carbon dioxide.When adding a mineral acid, the acidification is suitably carried out intwo steps, a first step to a pH of about 8 to 9, whereupon a certainripening, i.e., a particle growth, is allowed to occur before furtheracidification to a pH of from about 1 to about 4. However, it ispreferred that the acidification is carried out by means of an acidcation exchanger which, among other things, lead to more stableproducts. The acidification is preferably carried out by means of astrongly acid cation exchange resin, for example of sulfonic acid type.It is preferred that the acidification is carried out to a pH of fromabout 2 to 4, most preferably from about 2.2 to 3.0. The productobtained, an acid sol or polysilicic acid, contains silica-basedparticles with a high specific surface area, normally above 1000 m²gSiO₂ and usually around about 1300 to 1500 m²/g SiO₂.

[0018] The acid sol is then subjected to alkalisation, herein referredto as a first alkalisation step. The first alkalisation can be carriedout by addition of conventional alkali, e.g. lithium hydroxide, sodiumhydroxide, potassium hydroxide, ammonium hydroxide and mixtures thereof,and/or an aqueous silicate solution as defined above. Potassium andsodium water glass, particularly sodium water glass, with a molar ratioof SiO₂ to M₂O as defined above, is suitably used in the alkalisationstep. The SiO₂ content of the water glass solutions used for the firstalkalisation is suitably within the range of from about 3 to about 35%by weight and preferably within the range of from 5 to 30% by weight.The first alkalisation is usually carried out to a pH of at least 6,suitably at least 7 and preferably at least 7.5, and the pH is usuallyup to 10.5, suitably up to 10.0. The first alkalisation is furthersuitably carried out to a final molar ratio of SiO₂ to M₂O, M being asdefined above, of less than 100:1, suitably within the range of fromabout 20:1 to about 80:1, preferably from 30:1 to 70:1. In thepreparation of a sol having a low S-value as defined above the degree ofmicrogel can be influenced in several ways and be controlled to adesired value. The degree of microgel can be influenced by salt content,by adjustment of the concentration in the preparation of the acid soland in the first alkalisation step since in this step the degree ofmicrogel is influenced when the stability minimum for the sol is passed,at a pH of about 5. By prolonged times at this passage the degree ofmicrogel can be directed to the desired value. It is particularlysuitable to control the degree of microgel by adjustment of the drycontent, the SiO₂ content, in the first alkalisation step whereby ahigher dry content gives a lower S-value. By keeping the SiO₂ content inthe first alkalisation step within the range of from 4.5 to 8% by weightthe S-value can be controlled to the desired values of, for example,from 10 to 45%. To obtain sols with S-values within the range of from 20to 40% the SiO₂ content in the first alkalisation step is suitably keptwithin the range of from 5.0 to 7.5% by weight.

[0019] The silica-based particles present in the alkalised sol obtainedin the first alkalisation step is then subjected to particle growth sothat particles with a lower specific surface area and higher stabilityare obtained. The particle growth process should suitably be carried outto provide silica-based particles with a specific surface area of atleast 300 m²/g SiO₂, preferably at least 400 m²/g SiO₂, and mostpreferably at least 550 m²/g SiO₂, and up to about 1200 m²/g SiO₂,suitably less than 1000 m²/g SiO₂, notably up to 950 m²/g SiO₂. In apreferred embodiment of this invention, the particle growth process iscarried out to provide particles with a specific surface area within therange of from 550 to 725 SiO₂, preferably from 575 to 700 m²/g SiO₂. Inanother preferred embodiment of this invention, is carried out toprovide particles with a specific surface area of within the range offrom 775 to 1200 m²/g SiO₂, preferably from 800 to less than 1000 m²/gSiO₂. The decrease in surface area can be obtained by storage at roomtemperature during somewhat longer times, a day up to about two days andnights, or, preferably, by heat treatment. In the heat treatment, timesand temperatures can be adjusted so that shorter times are used athigher temperatures. Even if it of course is possible to use fairly hightemperatures during very short times it is, from a practical point ofview, more suitable to use lower temperatures during somewhat longertimes. In the heat treatment, the alkalised sol should suitably beheated at a temperature of at least 30° C., suitably from 35 to 95° andpreferably from 40 to 80° C. The heat treatment should suitably becarried out for at least 10 minutes, suitably from 15 to 600 minutes andpreferably from 20 to 240 minutes.

[0020] After the particle growth step, and optional cooling, theobtained silica-based sol is again subjected to alkalisation, hereinreferred to as a second alkalisation step, which further increases thepH. The second alkalisation can be carried out by addition ofconventional alkali, e.g. lithium hydroxide, sodium hydroxide, potassiumhydroxide, ammonium hydroxide and mixtures thereof, and/or an aqueoussilicate solution as defined above. Potassium and sodium water glass,particularly sodium water glass, with a molar ratio of SiO₂ to M₂O asdefined above, is suitably used in the second alkalisation step. TheSiO₂ content of the water glass solutions used for the secondalkalisation is suitably within the range of from about 3 to about 35%by weight and preferably within the range of from 5 to 30% by weight.The second alkalisation is suitably carried out to a pH of at least 8.0,suitably at least 10.0, preferably at least 10.5 and most preferably atleast 10.6. The pH can be up to about 11.5, suitably up to 11.0. Thesecond alkalisation is further suitably carried out to a final molarratio of SiO₂ to M₂O, M being as defined above, within the range of fromabout 10:1 to 100:1 and suitably from 12:1 to 40:1, preferably from 15:1to 30:1.

[0021] In a preferred embodiment of the invention, the process alsocomprises concentration of the silica-based sol. Concentration can becarried out after the second alkalisation. Alternatively, oradditionally, the alkalised sol obtained after the first alkalisationbut before the particle growth or heat treatment step, or the solobtained after the particle growth or heat treatment step but before thesecond alkalisation, can be subjected to concentration. Concentrationcan be carried out in known manner such as, for example, by osmoticmethods, evaporation and ultrafiltration. The concentration is suitablycarried out to produce silica contents of at least 10% by weight,preferably from 10 to 30% by weight, and more preferably from 12 to 25%by weight. The concentration is further suitably carried out so that thesilica-based sol obtained in the process has a specific surface area ofat least 80 m²/g aqueous sol, i.e., based on the weight of aqueous sol,preferably at least 85 m²/g aqueous sol, more preferably at least 90m²/g aqueous sol and most preferably at least 95 m²/g aqueous sol. In apreferred embodiment of this invention, the specific surface area of theaqueous silica-based sols obtained is suitably at least 115 m²/g aqueoussol preferably at least 120 m²/g aqueous sol. Generally, the specificsurface area of the aqueous sol obtained can be up to about 200 m²/gaqueous sol, suitably up to 150 m²/g aqueous sol and preferably up to130 m²/g aqueous sol.

[0022] If desired, the silica-based sols and particles can be modifiedby addition of compounds containing, for example, aluminium and/orboron. Suitable aluminium-containing compounds include aluminates likesodium aluminate and potassium aluminate, suitably sodium aluminate. Thealuminium-containing compound is suitably used in the form of an aqueoussolution. Suitable boron-containing compounds include boric acid,borates like sodium and potassium borate, suitably sodium borate,tetraborates like sodium and potassium tetraborate, suitably sodiumtetraborate, and metaborates like sodium and potassium metaborate. Theboron-containing compound is suitably used in the form of an aqueoussolution.

[0023] When using an aluminium-containing compound in the process, it issuitable to add it to the sol subjected to particle growth or heattreatment, either before or after the second alkalisation step.Alternatively, or additionally, the aluminium-containing compound can beadded to the silicate solution to be acidified, to the acid sol or tothe alkalised sol obtained in the first alkalisation step before theparticle growth or heat treatment step. The aluminium-containingcompound can be added in admixture with acid in the acidification stepand in admixture with alkali or silicate solution in any of thealkalisation steps. The aluminium-containing compound is suitably addedin an amount such that the obtained sol has a molar ratio of Al₂O₃ toSiO₂ as defined above.

[0024] When using a boron-containing compound in the process, it issuitable to add it to the sol subjected to particle growth or heattreatment, either before or after the second alkalisation step.Alternatively, or additionally, the boron-containing compound can beadded to the silicate solution to be acidified, to the acid sol or tothe alkalised sol obtained in the first alkalisation step before theparticle growth or heat treatment step. The boron-containing compoundcan be added in admixture with acid in the acidification step and inadmixture with alkali or silicate solution in any of the alkalisationsteps. The boron-containing compound is suitably added in an amount suchthat the obtained sol has a molar ratio of B to SiO₂ as defined above.If both aluminium-containing and boron-containing compounds are used,they are suitably added in amounts such that the obtained sol has amolar ratio of Al to B as defined above.

[0025] If the sol, before any aluminium and/or boron modification,contains too high amounts of alkali metal ions or ammonium ions, it ispreferable to remove at least part of these ions, for example by ionexchange, to provide silica-based sols with a final molar ratio of SiO₂to M₂O within the desired range as defined above.

[0026] According to the present process, silica-based sols andparticles, notably stable silica-based sols and particles, having theabove characteristics can be prepared and the produced sols exhibit goodstorage stability and can be stored for several months without anysubstantial decrease of the specific surface area and without gelation.

[0027] The silica-based sols and particles according to this inventionare suitable for use as flocculating agents, for example in theproduction of pulp and paper, notably as drainage and retention aids,and within the field of water purification, both for purification ofdifferent kinds of waste water and for purification specifically ofwhite water from the pulp and paper industry. The silica-based sols andparticles can be used as flocculating agents, notably as drainage andretention aids, in combination with organic polymers which can beselected from anionic, amphoteric, non-ionic and cationic polymers andmixtures thereof, herein also referred to as “main polymer”. The use ofsuch polymers as flocculating agents and as drainage and retention aidsis well known in the art. The polymers can be derived from natural orsynthetic sources, and they can be linear, branched or cross-linked.Examples of generally suitable main polymers include anionic, amphotericand cationic starches, anionic, amphoteric and cationic guar gums, andanionic, amphoteric and cationic acryl-amide-based polymers, as well ascationic poly(diallyidimethyl ammonium chloride), cationic polyethyleneimines, cationic polyamines, polyamidoamines and vinylamide-basedpolymers, melamine-formaldehyde and urea-formaldehyde resins. Suitablythe silica-based sols are used in combination with at least one cationicor amphoteric polymer, preferably cationic polymer. Cationic starch andcationic polyacrylamide are particularly preferred polymers and they canbe used singly, together with each other or together with otherpolymers, e.g. other cationic polymers or anionic polyacrylamide. Themolecular weight of the main polymer is suitably above 1,000,000 andpreferably above 2,000,000. The upper limit is not critical; it can beabout 50,000,000, usually 30,000,000 and suitably about 25,000,000.However, the molecular weight of polymers derived from natural sourcesmay be higher.

[0028] When using the present silica-based sols and particles incombination with main polymer(s) as mentioned above, it is furtherpreferred to use at least one low molecular weight (hereinafter LMW)cationic organic polymer, commonly referred to and used as anionic trashcatchers (ATC). ATC's are known in the art as neutralizing and/or fixingagents for detrimental anionic substances present in the stock and theuse thereof in combination with drainage and retention aids oftenprovide further improvements in drainage and/or retention. The LMWcationic organic polymer can be derived from natural or syntheticsources, and preferably it is an LMW synthetic polymer. Suitable organicpolymers of this type include LMW highly charged cationic organicpolymers such as polyamines, polyamideamines, polyethyleneimines, homo-and copolymers based on diallyl-dimethyl ammonium chloride,(meth)acrylamides and (meth)acrylates. In relation to the molecularweight of the main polymer, the molecular weight of the LMW cationicorganic polymer is preferably lower; it is suitably at least 1,000 andpreferably at least 10,000. The upper limit of the molecular weight isusually about 700,000, suitably about 500,000 and usually about 200,000.Preferred combinations of polymers that can be co-used with thesilica-based sols of this invention include LMW cationic organic polymerin combination with main polymer(s), such as, for example, cationicstarch and/or cationic polyacrylamide, anionic polyacrylamide as well ascationic starch and/or cationic polyacrylamide in combination withanionic polyacrylamide.

[0029] The components of the drainage and retention aids according tothe invention can be added to the stock in conventional manner and inany order. When using drainage and retention aids comprisingsilica-based particles and an organic polymer, e.g. a main polymer, itis preferred to add the polymer to the stock before adding thesilica-based particles, even if the opposite order of addition may beused. It is further preferred to add the main polymer before a shearstage, which can be selected from pumping, mixing, cleaning, etc., andto add the silica-based particles after that shear stage. LMW cationicorganic polymers, when used, are preferably introduced into the stockprior to introducing the main polymer. Alternatively, the LMW cationicorganic polymer and the main polymer can be introduced into stockessentially simultaneously, either separately or in admixture, forexample as disclosed in U.S. Pat. No. 5,858,174, which is herebyincorporated herein by reference. The LMW cationic organic polymer andthe main polymer are preferably introduced into the stock prior tointroducing the silica-based particles.

[0030] In a preferred embodiment of this invention, the silica-basedsols and particles are used as drainage and retention aids incombination with at least one organic polymer, as described above, andat least one aluminium compound. Aluminium compounds can be used tofurther improve the drainage and/or retention performance of stockadditives comprising silica-based particles. Suitable aluminium saltsinclude alum, aluminates, aluminium chloride, aluminium nitrate andpolyaluminium compounds, such as polyaluminium chlorides, polyaluminiumsulphates, polyaluminium compounds containing both chloride and sulphateions, polyaluminium silicate-sulphates, and mixtures thereof. Thepolyaluminium compounds may also contain other anions, for exampleanions from phosphoric acid, organic acids such as citric acid andoxalic acid. Preferred aluminium salts include sodium aluminate, alumand polyaluminium compounds. The aluminium compound can be added beforeor after the addition of the silica-based particles. Alternatively, oradditionally, the aluminium compound can be added simultaneously withthe silica-based sol at essentially the same point, either separately orin admixture with it, for example as disclosed by U.S. Patent No5,846,384 which is hereby incorporated herein by reference. In manycases, it is often suitable to add an aluminium compound to the stockearly in the process, for example prior to the other additives.

[0031] The components of the drainage and retention aids according tothe invention are added to the stock to be dewatered in amounts whichcan vary within wide limits depending on, inter alia, type and number ofcomponents, type of furnish, filler content, type of filler, point ofaddition, etc. Generally the components are added in an amount that givebetter drainage and/or retention than is obtained when not adding thecomponents. The silica-based sols and particles are usually added in anamount of at least 0.001% by weight, often at least 0.005% by weight,calculated as SiO₂ and based on dry stock substance, i.e. cellulosicfibres and optional fillers, and the upper limit is usually 1.0% andsuitably 0.5% by weight. The main polymer is usually added in an amountof at least 0.001%, often at least 0.005% by weight, based on dry stocksubstance, and the upper limit is usually 3% and suitably 1.5% byweight. When using an LMW cationic organic polymer in the process, itcan be added in an amount of at least 0.05%, based on dry substance ofthe stock to be dewatered. Suitably, the amount is in the range of from0.07 to 0.5%, preferably in the range from 0.1 to 0.35%. When using analuminium compound in the process, the total amount introduced into thestock to be dewatered depends on the type of aluminium compound used andon other effects desired from it. It is for instance well known in theart to utilise aluminium compounds as precipitants for rosin-basedsizing agents. The total amount added is usually at least 0.05%,calculated as Al₂O₃ and based on dry stock substance. Suitably theamount is in the range of from 0.1 to 3.0%, preferably in the range from0.5 to 2.0%.

[0032] Further additives which are conventional in papermaking can ofcourse be used in combination with the additives according to theinvention, such as, for example, dry strength agents, wet strengthagents, optical brightening agents, dyes, sizing agents like rosin-basedsizing agents and cellulose-reactive sizing agents, e.g. alkyl andalkenyl ketene dimers and ketene multimers, alkyl and alkenyl succinicanhydrides, etc. The cellulosic suspension, or stock, can also containmineral fillers of conventional types such as, for example, kaolin,china clay, titanium dioxide, gypsum, talc and natural and syntheticcalcium carbonates such as chalk, ground marble and precipitated calciumcarbonate.

[0033] The process of this invention is used for the production ofpaper. The term “paper”, as used herein, of course include not onlypaper and the production thereof, but also other cellulosicfibre-containing sheet or web-like products, such as for example boardand paperboard, and the production thereof. The process can be used inthe production of paper from different types of suspensions ofcellulose-containing fibres and the suspensions should suitably containat least 25% by weight and preferably at least 50% by weight of suchfibres, based on dry substance. The suspension can be based on fibresfrom chemical pulp such as sulphate, sulphite and organosolv pulps,mechanical pulp such as thermomechanical pulp, chemo-thermomechanicalpulp, refiner pulp and groundwood pulp, from both hardwood and softwood,and can also be based on recycled fibres, optionally from de-inkedpulps, and mixtures thereof. The pH of the suspension, the stock, can bewithin the range of from about 3 to about 10. The pH is suitably above3.5 and preferably within the range of from 4 to 9.

[0034] The invention is further illustrated in the following Exampleswhich, however, are not intended to limit the same. Parts and % relateto parts by weight and % by weight, respectively, unless otherwisestated.

EXAMPLE 1

[0035] A standard silica sol was prepared as follows:

[0036] 762.7 g sodium water glass with a molar ratio of SiO₂ to Na₂O of3.3 and SiO₂ content of 27.1% was diluted with water to 3000 g yieldinga silicate solution (I) with a SiO₂ content of 6.9% by weight. 2800 g ofthis silicate or water glass solution was passed through a column filledwith a strong cation exchange resin saturated with hydrogen ions. 2450 gof ion-exchanged water glass or polysilicic acid (II) with an SiO₂content of 6.5% by weight and a pH of 2.4 was collected from the ionexchanger. 1988 g of the polysilicic acid (II) was fed into a reactorand diluted with 12.3 g water. 173.9 g of the 6.9% silicate solution (I)was then added under vigorous agitation. The resulting solution was thenheated at 85° C. for 60 minutes and then cooled to 20° C. The obtainedsilica sol (1a) had the following characteristics:

[0037] Sol 1a (ref.): SiO₂ content=7.3% by weight, molar ratioSiO₂/Na₂O=40, pH=10.2, S-value=29%, viscosity=2.2 cP, specific surfaceareas=530 m²/g SiO₂ and 39 m²/g aqueous sol.

[0038] Two further silica sols, Sol 1b and Sol 1c, were produced whichhad the following characteristics:

[0039] Sol 1b (ref.): SiO₂ content=7.3% by weight, molar ratioSiO₂/Na₂O=63, pH=10.0, S-value=26%, viscosity=2.7 cP, specific surfaceareas=500 m²/g SiO₂ and 36.5 m²/g aqueous sol

[0040] Sol 1c (ref.): SiO₂ content=5.4% by weight, molar ratioSiO₂/Na₂O=35, pH=9.8, S-value=32%, viscosity=1.6 cP, specific surfaceareas=690 m²/g SiO₂ and 37 m²/g aqueous sol.

EXAMPLE 2

[0041] Six sols of silica-based particles according to the inventionwere prepared from a polysilicic acid similar to the polysilicic acid(II) produced with the same ion exchange process and with an SiO₂content of 5.46% by weight. To 102.0 kg of the polysilicic acid wasadded 1.46 kg of sodium water glass with a ratio SiO₂/Na₂O of 3.3 undervigorous agitation resulting in a solution with a molar ratio SiO₂/Na₂Oof 54.0. This solution was heat treated at 60° C. for 2 h 20 min andcooled to 20° C. whereupon the product was concentrated to a SiO₂content of 15.6% by weight. This intermediate sol product was nowdivided into six separate samples, a to f. Samples a to c were furtheralkalised with NaOH, samples d to f with water glass, to achieve solswith a molar ratio SiO₂/Na₂O between 21.5 and 34.0 and a silica contentof about 15.0% by weight. The obtained sols of silica-based particleshad the characteristics set forth in Table 1: TABLE 1 Vis- Molar ratioS-value cosity Specific Surface Areas Sol [SiO₂/Na₂O] pH [%] [cp] [m²/gSiO₂] [m²/g aq. sol] Sol 21.5 10.7 31 17 720 108.0 2a Sol 28.0 10.3 3029 710 106.5 2b Sol 34.0 10.0 29 40 690 103.5 2c Sol 21.5 10.7 31 20 680102.0 2d Sol 28.0 10.3 29 34 670 100.5 2e Sol 33.0 10.0 29 38 680 102.02f

EXAMPLE 3

[0042] A polysilicic acid (II) produced with the above ion exchangeprocess and alkalised with water glass to a molar ratio SiO₂/Na₂O of54.0 as in Example 2 was heat treated at 60° C. for 1 h. To 58 kg ofthis product was added 7.25 kg of diluted water glass with a molar ratioSiO₂/Na₂O of 3.3 and silica content 5.5% by weight. The resulting sol ofsilica-based particles, Sol 3, was concentrated to a silica content of15.2% by weight and had a molar ratio SiO₂/Na₂O=24, pH=10.7, S-value=34,viscosity=9.0 cp and specific surface areas=760 m²/g SiO₂ and 115.5 m²/gaqueous sol.

EXAMPLE 4

[0043] 1000 g polysilicic acid (II) with an SiO₂ content of 5.5% byweight was mixed with 14.5 g water glass solution with an SiO₂ contentof 27.1% by weight and a molar ratio SiO₂/Na₂O=3.3 under vigorousagitation resulting in a product with a molar ratio SiO₂/Na₂O of 51 anda silica content of 5.8% by weight SiO₂, which was heat treated at 60°C. for 1.5 h and then concentrated to a silica content of 16.7% byweight SiO₂, 283 g of the product obtained was mixed with 33.0 g NaOHresulting in a sol of silica-based particles, Sol 4, with SiO₂content=15.2% by weight, molar ratio SiO₂/Na₂O=21, pH=10.6, S-value=32%,viscosity=14.2 cP and specific surface areas=720 m²/g SiO₂ and 109.4m²/g aqueous sol.

EXAMPLE 5

[0044] The general procedure according to Example 3 was followed exceptthat the heat treatment was carried for 1.25 h and concentration wascarried out to higher silica contents. Two sols of silica-basedparticles were prepared; Sol 5a and Sol 5b. Sol 5a had SiO₂ content=18%by weight, molar ratio SiO₂/Na₂O=18, pH=10.7, S-value=36%, viscosity=18cP and specific surface areas=700 m²/g SiO₂ and 126 m²/g aqueous sol.Sol 5b had SiO₂ content=20% by weight, molar ratio SiO₂/Na₂O=18,pH=10.7, S-value=37%, viscosity=31 cP and specific surface areas=700m²/g SiO₂ and 140 m²/g aqueous sol.

EXAMPLE 6

[0045] Drainage performance was evaluated by means of a Dynamic DrainageAnalyser (DDA) available from Akribi, Sweden, which measures the timefor draining a set volume of stock through a wire when removing a plugand applying vacuum to that side of the wire opposite to the side onwhich the stock is present.

[0046] The stock used was based on a blend of 60% bleached birchsulphate and 40% bleached pine sulphate to which was added 30% groundcalcium carbonate as a filler. Stock volume was 800 ml, consistency0.25% and pH about 8.0. Conductivity of the stock was adjusted to 0.47mS/cm by addition of sodium sulphate.

[0047] In the tests, silica-based sols were used in conjunction with acationic polymer, Raisamyl 142, which is a conventional medium-highcationised starch having a degree of substitution of 0.042, which wasadded to the stock in an amount of 12 kg/tonne, calculated as dry starchon dry stock system. Silica-based sols according to Examples 1 to 4 weretested in this Example. In addition, Sols 6a and 6b were also tested forcomparison purposes. Sol 6a is a commercial silica sol with anS-value=45%, SiO₂ content=15.0% by weight, molar ratio SiO₂/Na₂O=40,viscosity=3.0 cP, specific surface areas =500 m²/g SiO₂ and 75 m²/gaqueous sol. Sol 6b is another commercial silica sol with anS-value=36%, SiO₂ content=10.0% by weight, molar ratio SiO₂/Na₂O=10,viscosity=2.5 cP, specific surface areas=880 m²/g SiO₂ and 88 m²/gaqueous sol. The silica-based sols were added in an amount of 0.5kg/ton, calculated as SiO₂ and based on dry stock system.

[0048] The stock was stirred in a baffled jar at a speed of 1500 rpmthroughout the test and chemical additions were conducted as follows: i)adding cationic starch to the stock following by stirring for 30seconds, ii) adding silica-based sol to the stock followed by stirringfor 15 seconds, iii) draining the stock while automatically recordingthe drainage time.

[0049] Drainage times for the different silica-based sols are shown inTable 2: TABLE 2 Dewatering time Silica-based sol [sec] Sol 1a (ref.)12.0 Sol 1b (ref.) 11.1 Sol 1c (ref.) 12.0 Sol 2d 9.7 Sol 3 9.5 Sol 49.4 Sol 6a (ref.) 12.0 Sol 6b (ref.) 9.8

EXAMPLE 7

[0050] Drainage performance was evaluated according to the generalprocedure of Example 6 except that the stock had a consistency of 0.3%and pH about 8.5. Retention performance was evaluated by means of anephelometer by measuring the turbidity of the filtrate, the whitewater, obtained by draining the stock.

[0051] Silica-based sols according to Example 5 according to theinvention were tested against Sol 6a used for comparison. Table 3 showsthe drainage time obtained at various dosages (kg/ton) of silica-basedparticles, calculated as SiO₂ and based on dry stock system. Theaddition of only cationic starch (12 kg/tonne, calculated as dry starchon dry stock system) resulted in a drainage time of 15.8 sec. TABLE 3Silica-based Drainage time (sec)/Turbidity (NTU) at SiO₂ dosage of sol0.5 kg/t 1.0 kg/t 1.5 kg/t 2.0 kg/t 3.0 kg/t Sol 6a 11.1/— 8.8/59 7.9/587.1/54 6.8/60 (ref.) Sol 5a  9.0/— 7.1/52 6.3/50 5.2/52 5.7/53 Sol 5b 8.9/— 6.9/— 6.3/— 5.7/— 6.0/—

1. Process for the production of an aqueous sol containing silica-basedparticles which comprises: (a) acidifying an aqueous silicate solutionto a pH of from 1 to 4 to form an acid sol; (b) alkalising the acid solat an SiO₂ content within the range of from 4.5 to 8% by weight to a pHof at least 7; (c) allowing particle growth of the alkalised sol for atleast 10 minutes; (d) alkalising the obtained sol to a pH of at least10.0; and (e) optionally concentrating the sol obtained according to(b), (c) or (d) to provide an aqueous sol containing silica-basedparticles and having a specific surface area of at least 90 m²/g aqueoussol.
 2. Process for the production of an aqueous sol containingsilica-based particles which comprises: (a) acidifying an aqueoussilicate solution to a pH of from 1 to 4 to form an acid sol; (b)alkalising the acid sol at an SiO₂ content within the range of from 4.5to 8% by weight; (c) heat-treating the alkalised sol at a temperature ofat least 30° C.; (d) alkalising the heat-treated sol to a pH of at least10.0; and (e) optionally concentrating the sol obtained according to(b), (c) or (d) to provide an aqueous sol containing silica-basedparticles and having a specific surface area of at least 90 m²/g aqueoussol.
 3. Process according to claim 1 or 2, characterised in that itcomprises (e) concentrating the sol obtained according to (c) or (d) toprovide a sol having a specific surface area of at least 95 m²/g aqueoussol.
 4. Process according to claim 1, 2 or 3, characterised in that thealkalisation according to (b) and (d) is carried out by means of anaqueous silicate solution.
 5. Process according to any of claims 1 to 4,characterised in that the particle growth and heat-treatment accordingto (c) is carried out at a temperature within the range of from 35 to95° C.
 6. Process according to any of claims 1 to 5, characterised inthat the particle growth and heat-treatment according to (c) is carriedout for 20 to 240 minutes.
 7. Process according to any of claims 1 to 6,characterised in that the alkalisation according to (d) produces a solhaving a molar ratio of SiO₂ to M₂O, where M is alkali metal orammonium, within the range of from 15:1 to 30:1 and a pH of at least10.6.
 8. Process according to any of claims 1 to 7, characterised inthat it further comprises addition of an aluminium-containing compoundand/or a boron-containing compound.
 9. Process according to any ofclaims 1 to 8, characterised in that the silica-based particles obtainedhave a specific surface area of at least 550 m²/g SiO₂.
 10. Aqueous solcontaining silica-based particles obtainable by a process according toany of claims 1 to
 9. 11. Aqueous sol containing silica-based particles,characterised in that it has a specific surface area of at least 115m²/g aqueous sol and the silica-based particles have a specific surfacearea of at least 550 and less than 1000 m²/g SiO₂.
 12. Aqueous solcontaining silica-based particles, characterised in that it has aspecific surface area of at least 115 m²/g aqueous sol and an S-valuewithin the range of from 10 to 45%.
 13. Aqueous sol according to claim11 or 12, characterised in that it has a molar ratio of SiO₂ to M₂O,where M is alkali metal or ammonium, within the range of from 15:1 to40:1.
 14. Aqueous sol according to claim 12 or 13, characterised in thatthe silica-based particles have a specific surface area of at least 550m²/g SiO₂.
 15. Aqueous sol according to any of claims 11 to 14,characterised in that it has an S-value within the range of from 25 to35%.
 16. Aqueous sol according to any of claims 11 to 15, characterisedin that it has a silica content of at least 10% by weight.
 17. Use of anaqueous sol containing silica-based particles according to any of claims10 to 16 or produced by a process according to any of claims 1 to 9 as adrainage and retention aid in the production of paper.
 18. Process forthe production of paper from an aqueous suspension containing cellulosicfibres, and optional fillers, which comprises adding to the suspensionsilica-based particles and at least one charged organic polymer, formingand draining the suspension on a wire, characterised in that thesilica-based particles are obtained by a process according to any ofclaims 1 to 9 or present in an aqueous sol according to any of claims 10to
 16. 19. Process for the production of paper which comprises: (a)providing an aqueous suspension containing cellulosic fibres, andoptional fillers; (b) providing an aqueous sol containing silica-basedparticles, the sol having a specific surface area of at least 90 m²/gaqueous sol and the silica-based particles having a specific surfacearea of less than 1000 m²/g SiO₂; (c) providing at least one chargedorganic polymer; (d) adding the charged organic polymer and thesilica-based particles to the suspension; (e) forming and draining theobtained suspension on a wire.
 20. Process for the production of paperwhich comprises: (a) providing an aqueous suspension containingcellulosic fibres, and optional fillers; (b) providing an aqueous solcontaining silica-based particles having a specific surface area of atleast 90 m²/g aqueous sol and an S-value within the range of from 10 to45%; (c) providing at least one charged organic polymer; (d) adding thecharged organic polymer and the silica-based particles to thesuspension; (e) forming and draining the obtained suspension on a wire.21. Process according to claim 19 or 20, characterised in that the solhas a specific surface area in the range of from 95 to 150 m²/g aqueoussol.
 22. Process according to claim 19, 20 or 21, characterised in thatthe silica-based particles have a specific surface area of at least 550m²/g SiO₂.
 23. Process according to any of claims 19 to 22,characterised in that the charged organic polymer is cationic starch orcationic polyacrylamide.
 24. Process according to any of claims 19 to23, characterised in that the aqueous sol is diluted to a silica contentof from 0.05 to 5% by weight before adding the silica-based particles tothe suspension.
 25. Process according to any of claims 19 to 23,characterised in that the silica-based particles are added to thesuspension in an amount of from 0.005 to 0.5% by weight, calculated asSiO₂ and based on dry cellulosic fibres and optional fillers.